Climate Driver Update history
Climate drivers in the Pacific, Indian and Southern oceans and the Tropics

Sea surface temperatures (SSTs) remain cooler than average in the central and eastern tropical Pacific for the week ending 11 February. The strength of these cool anomalies has declined further in the last fortnight.

Latest values for the week ending 11 February are: NINO3 −0.8 °C, NINO3.4 −0.6 °C, NINO4 −0.2 °C.

A marine heatwave with large positive SST anomalies persists across the Tasman Sea, with the strongest anomalies in waters around Tasmania. These shallow warm anomalies developed during the second half of November.

Positive SST anomalies are also present across the south Pacific. To the north of Australia SSTs are generally close to average, although weak warm anomalies persist across northern parts of the Maritime Continent.

Persistent NINO3 or NINO3.4 values cooler than −0.8 °C are typically indicative of La Niña, while persistent values warmer than +0.8 °C are typical of El Niño.

The La Niña in the Pacific Ocean continues to decline. Sea surface temperatures in the central tropical Pacific have warmed steadily since late December, with most models forecasting La Niña will end early in the southern hemisphere autumn.

El Niño–Southern Oscillation (ENSO) indicators continue to show at least some La Niña characteristics. Sea surface temperatures indicate a weak La Niña pattern, with the coolest waters concentrated in the eastern Pacific Ocean, while the Southern Oscillation Index is neutral, but weakly positive. However, a very strong pulse of the  Madden-Julian Oscillation (MJO), which drove a burst of monsoonal activity over northern Australia in late January, has caused the western Pacific trade winds to weaken considerably. The rapid weakening of the trade winds may hasten the decline of La Niña. The MJO also led to above-average cloudiness at the Date Line for the first time since early September 2017.

In order for 2017–18 to be classed as a La Niña year, thresholds need to be exceeded for at least three months. Four of the eight climate models surveyed by the Bureau suggest this event is likely to last at least until late summer, while a few continue the event into the southern hemisphere autumn of 2018.

Typically, the strength of a La Niña event reflects the strength of its impact upon Australian climate. The current event is weak, and hence climate patterns have been significantly different from those observed in the last strong La Nina of 2010–12.  Large parts of eastern Australia have been drier than average for the past two or three months, the opposite of what is typically expected during La Niña.

The Indian Ocean Dipole (IOD) is currently neutral. IOD events are unable to form between December and April.

Cloudiness near the Date Line has fluctuated around average since the start of February. The recent strong Madden-Julian Oscillation pulse led to above-average cloudiness at the Date Line in the last few days; the first time this has occurred since early September 2017. Cloudiness in this region had been generally below average between early August 2017 and the end of January 2018. The change in cloudiness is a sign La Niña is on the decline.

Equatorial cloudiness near the Date Line typically increases during El Niño (negative OLR anomalies) and decreases during La Niña (positive OLR anomalies).

Trade winds for the 5 days ending 11 February were stronger than average across the western equatorial Pacific, with a reversal (i.e. easterly winds replaced by westerly winds) in some areas south of the equator around 160°E.

The passage of a very strong pulse of the Madden-Julian Oscillation across the Maritime Continent has driven this westerly wind burst, and may continue to see weakened trade winds across the Pacific region during the next week. This is expected to hasten the decline of the La Niña.

During La Niña events, there is a sustained strengthening of the trade winds across much of the tropical Pacific, while during El Niño events there is a sustained weakening, or even reversal, of the trade winds.

All international climate models surveyed by the Bureau indicate that the current event is likely near its end.

Equatorial Pacific sea surface temperatures are likely to continue to warm over the coming months, although three of the eight surveyed models maintain values close to La Niña thresholds well into the southern hemisphere autumn. All models predict neutral NINO3.4 values for the southern hemisphere winter.

In order to consider 2017–18 a La Niña year, NINO3 or NINO3.4 values at least as cool as −0.8 °C need to be present for three months or more.

Sea surface temperature (SST) anomalies for January show SSTs were cooler than average in the central and eastern tropical Pacific Ocean and across large areas of the Pacific south of the equator and east of 110°W (i.e. the area to the west of South America). Generally weak warm anomalies were present across most of the remainder of the South Pacific, most of the western Pacific, and parts of the mid-latitudes of the North Pacific. Areas of stronger warm anomalies in excess of two degrees above average occurred in the Tasman Sea between southeastern Australia and New Zealand.

The January value for NINO3 was −0.8 °C, NINO3.4 −0.6 °C, and NINO4 −0.2 °C. NINO3 and NINO3.4 warmed slightly compared to December values, while NINO4 held steady.

The 30-day Southern Oscillation Index (SOI) to 11 February is +2.5 (90-day value +2.4), which is within the neutral range. While the SOI fluctuates more during the southern hemisphere summer due to movement of tropical systems, the SOI has maintained a steady decline over the past two weeks.

Sustained positive values of the SOI above +7 typically indicate La Niña while sustained negative values below −7 typically indicate El Niño. Values between +7 and −7 generally indicate neutral conditions.

The Indian Ocean Dipole (IOD) is neutral. The weekly index value to 11 February was −0.08 °C. All six of the climate models surveyed by the Bureau indicate that the IOD will remain neutral into the southern hemisphere winter of 2018.

The influence of the IOD on Australian climate is weak during December to April. This is because the monsoon trough shifts south over the tropical Indian Ocean changing wind patterns, which prevents the IOD pattern from being able to form.

The four-month sequence of sub-surface temperature anomalies (to January) shows the cool anomalies in the central and eastern equatorial Pacific Ocean have weakened during January, a typical feature of a decaying La Niña. Areas of cool anomalies between the surface and 150 m depth east of 150°W reached more than 2.5 °C cooler than average for January. In the west, sub-surface warm anomalies between around 100 and 150 m depth have also weakened in strength, and only exceeded 1 °C in a small area.

Sub-surface temperatures for the 5 days ending 11 February show a pool of warmer than average water in the western equatorial Pacific between 100 and 200 m below the surface. This region has warmed strongly over the past fortnight.

The development of warm sub-surface waters in the western equatorial Pacific is a typical precursor to the breakdown of a La Niña event, as these warmer temperatures can migrate east and erode the cool anomalies that support the event.

Temperatures in the sub-surface of the eastern tropical Pacific continue near average to just slightly cooler than average.